1. Field of the Invention
The present invention relates to a coating apparatus for a magnetic recording medium, e.g. a magnetic tape and a floppy disk, and more particularly, to the improvement of a die nozzle of the same.
2. Description of the Prior Art
In the manufacturing process of a magnetic recording medium, there is a step of coating a plastic film with a magnetic layer and a back layer by the use of a coating apparatus having a die nozzle. It is generally known that as best shown in FIG. 8, such a die nozzle 5 delivers from the front-end slit thereof a flow of magnetic coating liquid 7 which is continuously supplied by a pump 6 and in turn, applied onto a base film 8 which runs without being supported from the back, while the clearance between the die nozzle 5 and the base film 8 is maintained constant by the pressure of the magnetic coating liquid. The die nozzle 5 has at the film unloading side a rear lip 1 thereof provided with a convex surface and at the film loading side a front lip 2 thereof, both of which are arranged so that the coating liquid in a liquid pocket P defined by the rear lip 1 remains under pressure during the coating procedure, whereby high-speed and uniform coating operations will be ensured. Another advantage of the die nozzle 5 is that the convex shape in cross section of the rear lip 1 allows no foreign matters to accumulate on the surface and thus, uneven coating which commonly occurs in stripe form during continuous long-run operations will be avoided.
However, we have found through experiments that during the coating of a base film with a particular coating liquid, e.g. a magnetic coating liquid, which has a thixotropic property exhibiting a dependency of viscosity on a shear velocity, by the foregoing prior art coating apparatus, there is a possible tendency to have a lengthwise stripe pattern of slightly uneven coating appearing on the coating surface, which is distinguished from the aforementioned conventional stripe-pattern coating fault.
FIG. 7-a illustrates the result of measurements of a coating surface, measured by a 3-dimension surface profile analyzer, which was prepared by coating a 14-.mu.m-thick base film of polyethylene terephthalate with the magnetic coating liquid shown in Table 1 and then, finishing it with calendering. The resultant measurements on the coating surface are shown for clarity in the form of a graphic diagram in which the projections represent regions higher than the average value which is assigned as 0. As shown, the coating surface exhibits a stripe pattern of about 300 .mu.m pitch coating inequality extending in the lengthwise direction of the base film, which is clearly different from that caused by the accumulation of foreign matters during the coating by the conventional coating apparatus. The average roughness on the coating surface (expressed by an RMS value hereinafter) shown in FIG. 7-a was 11.5 nm. The results measured in the magneto electric conversion using an MII format deck were -2 dB in the video frequency output (of 7 MHz) and -1 dB in the S/N ratio, as compared with our reference tape. This means that the stripe pattern on the coating surface shown in FIGS. 7-a causes a considerable decline in the magneto electric conversion.
We have also found through experiments the cause of the development of such a lengthwise stripe pattern, which will now be described. In magnetic coating liquid, particles of magnetic powder which are commonly present, likely in the form of primary particles, but forming a three-dimensional network structure due to the magnetic forces between the particles, will be converted into agglomerates having a certain size when a shear stress is applied to the structure (as depicted in a Japanese magazine "Tosoh Kogaku Coating Technology", Vol. 21, No. 10 issued in 1986). Particularly, in a magnetic coating liquid which contains powder of magnetic material having a small lengthwise diameter of the average particle and offering a high magnetic force, the presence of such agglomerates is of an order of several tens to a hundred micrometers during movement of the liquid which is thus increased in agglomeration. During the application of the highly agglomerative magnetic coating liquid onto a base film from the die nozzle, the agglomerates are forced out from the liquid pocket P, shown in FIG. 8, onto the rear lip, thus developing an unwanted stripe pattern.
It was also found out through experiments that the greater the magnetic force and the smaller the lengthwise diameter of average particle of the magnetic powder in a magnetic coating liquid, the more the stripe pattern of uneven coating appears.
Corresponding to the high-density recording on a magnetic recording medium, ultrafine magnetic powder exhibiting a high magnetic force has recently been employed. However, such a stripe pattern of uneven coating on the coating surface as described above tends to degrade the characteristics of magneto electric conversion including a video frequency output and an S/N ratio and thus, will cause a substantial defect in the product quality.
TABLE 1 ______________________________________ Iron powder 100 parts Lengthwise diameter of particle: 0.18 .mu.m Average diameter ratio: 1:12 .sigma.s 128 emu/g Hc 1530 Oe Conductive carbon 2 parts Polyurethane resin 10 parts Vinyl chloride vinyl acetate copolymer 10 parts Alumina (.alpha.-Al.sub.2 O.sub.3) 5 parts Stearic acid 2 parts Butyl stearate 2 parts Methyl ethyl ketone 200 parts Toluene 180 parts Cyclohexanone 40 parts ______________________________________